Monoamine transporters play a variety of roles, and compounds with affinity for the monoamine transporters have been proposed for therapy and/or diagnosis of medical indications that include (but are not limited to) attention deficit hyperactivity disorder (ADHD), Parkinson's disease, cocaine addiction, smoking cessation, weight reduction, obsessive-compulsive disorder, various forms of depression, traumatic brain injury, stroke, and narcolepsy. Examples of monoamine transporters include, e.g., the dopamine transporter (DAT), serotonin transporter (SERT) or norepinephrine transporter (NET).
Therapies for treating diseases and disorders related to monoamine transport are needed. For example, there is a need for protective agents for neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease as well as therapeutic agents for dopamine related dysfunction such as Attention Deficit Disorder. Compounds that inhibit monoamine reuptake in the mammalian system are sought to provide such therapies.
Other neuropsychiatric disorders, including Tourette's Syndrome and Lesch Nyhan Syndrome and possibly Rett's syndrome, are also marked by changes in DAT density. The DAT also is the target of the most widely used drug for attention deficit disorder, methylphenidate. The capacity to monitor the transporter in persons suffering from this disorder can have diagnostic and therapeutic implications.
The density of the DAT in the brains of substance abusers has also been shown to deviate from that in normal brain. For example, the density is elevated in post-mortem tissues of cocaine abusers (Little et al., Brain Res. 1993, 628, 17-25). On the other hand, the density of the DAT in chronic nonviolent alcohol abusers is decreased markedly. (Tiihonen et al., Nature Medicine 1995, 1, 654-657). Brain imaging of substance abusers can be useful for understanding the pathological processes of cocaine and alcohol abuse and monitoring restoration of normal brain function during treatment.
Accordingly, compounds that bind to the DAT provide important clinical information to assist in the diagnosis and treatment of these and other DAT related disease states.
Serotonin (5-hydroxytryptamine) neurotransmission is regulated and terminated by active transport via the serotonin transporter (SERT). Inhibition of 5-hydroxytryptamine reuptake has an effect on diseases mediated by 5HT receptors. Compounds that provide such inhibition can be useful, for example, as therapeutic anti-depressants. Structurally related to dopamine and norepinephrine transporters (Nelson N. 1998. J Neurochem 71:1785-1803), the SERT is the primary site of action of diverse antidepressant drugs, ranging from tricyclics such as imipramine and amitriptyline, to serotonin selective reuptake inhibitors (SSRI's) such as citalopram, fluoxetine and sertraline.
Antidepressant drugs delay the removal of extracellular serotonin from the synapse by blocking serotonin transport, thereby prolonging the duration of serotonin receptor activity. The increased availability of serotonin triggers a cascade of neuroadaptive processes, which produces symptom relief after two to four weeks. Presently known antidepressants also produce certain side effects and may selectively alleviate specific symptoms of depression (Nestler E J. 1998. Biol Psychiatry 44:526-533). Thus, it is desirable to develop novel antidepressants. The majority of clinically approved drugs to treat depression or obsessive-compulsive disorder are high affinity inhibitors of serotonin and/or norepinephrine transport. Of these transporter inhibitors, none are tropane analogs.
Norepinephrine regulates mood, is involved in learning and memory, and controls endocrine and autonomic functions. Dysfunction of norepinephrine neurotransmission has been implicated in depression, cardiovascular and thermal pathophysiology. The norepinephrine transporter (NET) regulates extracellular levels of norepinephrine in brain, in heart, and in the sympathetic nervous system. Clinically, the norepinephrine transporter is a principal target of selective or non-selective anti-depressant drugs and stimulant drugs of abuse such as cocaine and amphetamines. Blockade of the norepinephrine transporter is implicated in appetite suppression. Gehlert et al. J. Pharmacol. Exp. Ther. 287:122-127 (1998). Imaging of the norepinephrine transporter may also be useful for viewing the status of sympathetic innervation in the heart and in other adrenergic terminals, and for detecting neuroblastomas. Hadrich et al. J. Med. Chem. 42:3010-3018 (1999); Raffel et al., J. Nucl. Med. 40:323-330 (1999).
Monoamine transporters such as, the dopamine transporter, serotonin transporter and norepinephrine transporter, are localized on monoamine nerve terminals. Compounds that bind to these sites can be useful as (i) probes for neuro-degenerative diseases (e.g., Parkinson's disease), (ii) therapeutic drugs for neurodegenerative diseases (e.g., Parkinson's and Alzheimer's disease), (iii) therapeutic drugs for dopamine dysfunction (e.g., Attention Deficit Disorder), (iv) treatment of psychiatric dysfunction (e.g., depression) and (v) treatment of clinical dysfunction (e.g., migraine).
It is desirable to avoid unwanted side effects of treatments targeting monoamine transporters, to the extent possible. It is also desirable to produce efficient and effective diagnostics for various conditions involving monoamine transporters.
Furthermore, it would be useful to improve the bioavailability of compounds used to treat and/or diagnose monoamine transporter related diseases and disorders. It would be useful to modify these compounds to block or reduce metabolism of the compounds, while maintaining, or ideally, improving potency and/or selectivity of the compounds.
It would also be useful to have compounds that are prodrugs, i.e., a compound that undergoes chemical conversion by an organism or enzyme preparation before exhibiting its pharmacological effects. Prodrugs are drugs containing specialized non-toxic protective groups used in a transient manner to alter or to eliminate undesirable properties in the parent molecule.